The present invention relates to an electrode assembly which is sealingly fitted in an arc tube forming a light source of a discharge lamp unit. More particularly, the invention relates to an electrode assembly for an arc tube in which a molybdenum foil is connected to an electrode bar by spot welding, and to a method for manufacturing the electrode assembly.
FIG. 5 is a sectional view of an arc tube. The arc tube 1 includes a pair of electrode assemblies 3 inserted into a glass tube, the latter being sealed at both ends to form pinch-seal portions 2b which sealingly fix the electrode assemblies 3, and a closed glass ball 2b between the pinch-seal portions 3. The electrode assemblies 3 have respective electrode bars 5 which protrude into the glass ball 2b, and lead wires 6 which extend to the outside from the respective pinch-seal portion 2a. More specifically, each of the electrode assemblies 3, as shown in FIG. 5, includes an elongated molybdenum foil 4. The electrode bar 5 is welded to one end of the molybdenum foil 4, and the lead wire 6 is welded to the other end of the molybdenum foil 4. In FIG. 6, reference numeral 8 designates spot-welded regions of the electrode assembly.
In manufacturing the electrode assembly 3, as shown in FIG. 7, the molybdenum foil 4 and the electrode bar 5 are overlapped with each other, and then spot-welded with the overlapped portions 7 clamped with spot-welding electrodes 8aand 8b. In general, the electrode bar is made of tungsten, and therefore its weldability with the molybdenum foil is not high. Hence, the welding operation is carried out with the electrode bar 5 and the molybdenum foil 4 pressed against each other. As a result, sometimes the soft molybdenum foil 4 is scratched by the rectangular corner 5a.sub.1 of the end portion 5a of the electrode bar 5, and thus it can be cracked. Since the electrode bar 5 is formed by cutting a belt-shaped electrode bar material into a predetermined length, the electrode bar 5 may have burrs at the end 5a.sub.1, or acute-angled cut ends. In such a case, the above-described difficulty in the manufacture of the electrode assembly is especially prevalent. If the molybdenum foil 4 is cracked, then, in the pinch-sealing step employed in manufacturing the arc tube, the molybdenum foil may tear, as shown in FIG. 8.